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Photo-induced dynamical changes in amorphous As2S3 films
Solid State Communications, Vol. 34, pp. 201-204. Pergamon Press Ltd. 1980. Printed in Great Britain. PHOTO-INDUCED DYNAMICAL CHANGES IN AMORPHOUS As& FILMS K. Tanaka Department of Engineering Science, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received 27 November 1979 by W. Sasaki)
Optical properties of amorphous AsaSs films, which have been illuminated well by bandgap light in advance, can be changed dynamically by exposing to less-bandgap light. This dynamical change has been studied in connection with its plausible relation to the reversible photo-induced change. It has been found that these changes have intimate connections with each other, and can be explained by a certain configurational dia_ gram in a coherent fashion.
2. EXPERIMENTS
1. INTRODUCTION IN AMORPHOUS CHALCOGENIDE FILMS, photostructural transformations [l] have been extensively studied so far. It is well known that the transformations are induced by bandgap illumination, and are associated with certain changes in optical properties, i.e. so-called photodarkening and related refractive-index changes. DeNeufville [I] has classified the phenomena into reversible and irreversible processes, according to whether a heat treatment can or cannot restore the initial structural states. Origin of the irreversible change has been supposed to be a kind of photo-polymerization process [l-3] , whereas the details of the reversible are still not understood. In a previous paper [4], the present author has reported that refractive index of an amorphous Ass& film, which has been illuminated sufficiently by bandgap light beforehand, is dynamically quenched by exposing to radiation with energy below the bandgap. The refractive index of the specimen quenched by the less-bandgap illumination can be enhanced by the bandgap illumination, and the quenching and enhancement can be repeated many times. On the basis of these observations, it has been supposed that the features of the dynamical refractive-index change can be accounted for essentially with a kinetics of some band-tail states. Characteristics of the reversible and dynamical changes are qualitatively similar, but for the fact that in the case of the former the change is recovered by a thermal annealing, whereas in the case of the latter by an optical exposure. Therefore, it is important to clarify the relationship between the reversible and dynamical photo-induced optical changes. This is a purpose of the present paper.
Amorphous AS& fdms were prepared by vacuumevaporation technique under conditions described previously [4]. The samples were, then, annealed at the reversible temperature [l] which is slightly below the glass transition temperature for the case of A& [5] . Prism-coupling technique was utilized for the measurements of refractive index at a photon energy of 1.96 eV and its dispersion [6]. Optical transmittance was investigated also in this study. 3. RESULTS AND DISCUSSION Figure 1 shows a typical result of refractive-index changes in an As& film. For exciting the sample, two light beams of photon energy of 1.96 and 2.8 1 eV having intensities of lo3 and 10’ W cmq2, respectively, were employed. The photon energies of the beams are lower or higher than optical bandgap energy Ego of about 2.4 eV [7]. In addition, it is noted that photon densities absorbed by the specimen are 102’ - 10z2 cmw3see-’ for both the beams. When the specimen is exposed to the 2.81 eVlight, the refractive index increases monotonically from no to a saturation value n1 as a result of the reversible photostructural change [ 1,4] . If the bandgap radiation is ceased, then a spontaneous relaxation to ns results [4] . The magnitude n2 is stable at least one month if the sample is stored in the dark. However, illumination of the less-bandgap light quenches the magnitude n2 to a steady state value n3. It should be noted here that the magnitude n3 is attained also with illumination of the 1.96 eV-light to the annealed specimen as shown by a dashed line in Fig. 1. Note that the magnitudes n2 and n3 are independent of the intensities of the light. As is shown in Fig. 2, the magnitudes of the dynamical refractive-index change n2 -n3 increase with
* This work was partially supported by the Ministry of Education Grant-in-Aid for Scientific Research. 201
202
DYNAMICAL CHANGES IN AMORPHOUS As& FILMS
Vol. 34, No. 3
//---/ “I.96 ..’
1.96eV
eV- light
2.5 I min
photon energy I
time
Fig. 1. Time dependence of photo-induced changes in refractive index by switching on and off the irradiations of 1.96 and 2.81 eV in an annealed As& film of 0.3 pm thick. A dashed line shows a change induced by 1.96 eV-light.
of probing
light
(eV)
Fig. 2. Dependences of n2 -n3 on the photon energy of probing light. Optical bandgap energy is indicated by -5~ -
IO2
2.81 or 1.96 eV A at 77°K
photon energy of excitotionleV)
8 photon
energy
(eV)
Fig. 3. Optical transmittances of an Ass& film after various treatments; annealing and exposure to 1.96 and 2.81 eV-light at room temperature and 77 K. The measurements are carried out at room temperature. The related refractive indices defined in Fig. 1 are written in. probing photon energy in a range of 1.5 - 2.2 eV. Therefore, it is expected [4] with the aid of Kramers-Kronig relations that related absorption charges exist mainly in the photon-energy region higher than 2.2 eV. Figure 3 shows transmission spectra of the specimen after various treatments. The effects induced by the illumination of 2.81 and 1.96 eV-light at room temperature are indicated by solid and dotted arrows, respectively. It is clear that the dynamical refractiveindex change from n2 to n3 accompanies a shift of the optical absorption edge to higher photon energy. Furthermore, the optical absorption edge shifts to lower photon energy in accord with the refractive-index change from no to n3. It should be mentioned that the modifications in the transmission spectra are parallel shifts of the absorption edge. These characteristics are qualitatively similar to those observed in the reversible
Fig. 4. Refractive-index changes at 1.96 eV and absorption-edge shifts with light exposure of various photon energies. The physical quantities on the vertical axes are plotted as the differences from those of the annealed specimen. photodarkening, which is the change induced by the bandgap illumination of the annealed film. As is shown in Figs. 1 and 3, the photon energy of the last exposure determines the film character completely, irrespective of the previous illumination history. That is, the optical properties of the film are determined uniquely by the photon energy of the last exposure. Figure 4 shows the dependence of the optical properties on the photon energy of the excitation. It is apparent that the refractive-index change and the absorption-edge shift are correlated well with each other. When the photon energy of the excitation is greater than the optical bandgap energy Ego, these are nearly independent of the photon energy of the excitation. Otherwise, the change and the shift decrease with the photon energy. It is noted that an annealing treatment at the reversible temperature for all the illuminated films produces a unique physical state, irrespective of the previous illumination history. It is also noted that, since absorption coefficients [7] become smaller with
Vol. 34, No. 3
DYNAMICAL CHANGES IN AMORPHOUS As& FILMS
configuration
Fig. 5. Proposed configurational diagram. photon energies, it seems practically impossible to obtain the data below 1.96 eV. It is known [ 1 ] that illumination of the bandgap light at 77 K reveals larger absorption-edge shift. It is interesting that the similar change is observed with the illumination of the 1.96 eV-light at 77 K as shown by a dashed line in Fig. 3. (Note that the measurements are performed at room temperature.) Therefore, it is supposed that the optical bleaching at room temperature with the irradiation of the less-bandgap light of the film, which was illuminated beforehand by the bandgap light, is assisted essentially with thermal energy of 20 w 30 meV. The effects induced by bandgap and less-bandgap illumination can be understood in terms of a configurational energy diagram illustrated in Fig. 5. The ground level consists of a double well X and Y, and the excited level has a single well 2. In the present speculation, it is essential that the minimum of the well 2 is located above the well Y. The wells X and Y are separated by a potential barrier of Es. Further, there is an energy difference ExY between minima of X and Y. These magnitudes are estimated as EB = 0.5 - 1 eV and ExY = 0.01 - 0.1 eV with results of annealing kinetics. It is supposed that E,, and EYZ N Ego as well as Exz E yz ~0.1 eV. In amorphous specimens, it is very natural to imagine that potential fluctuates locally, thus the precise shape and magnitude of the wells might vary from site to site. In this model, it is not always necessary that transition from Z to X or Y is radiative. For a specimen annealed at the reversible temperature, it is reasonable to assume that all the states are in the well X. According to the Frank-Condon principle, it is expected that the bandgap exposure of the
203
annealed film at low temperatures produces a transition x,, + jr -+ ze + y,, . Reexcitation of the states in the well Y is possible, however this proceeds asyo + i3 + z. +yo. Therefore, almost all the states are transferred from X to Y by the bandgap illumination at low temperatures. Considering the spread of wavefunctions at the well X and the fluctuation in the gap energy Exz, we suppose that illumination of the less-bandgap light at low temperatures produces similar processes with lower efficiencies. Thus, by the optical exposure at low temperatures, the gap decreases from Exz to EyZ. The ensemble average of the difference Exz - EYZ corresponds to the edge shift at 77 K in Fig. 3. On the contrary, the light exposure at room temperature has a distinct effect. With the aid of the thermal energy, equilibrium states in the well X and Y are not confined to the bottoms. In addition, since the relaxation time in the well Z is lo-l2 set and transition probability from Z to X or Y is lOa set-’ [8] , it is reasonable to assume a thermal equilibrium level for the well Z. These hypothetical levels are indicated tentatively by horizontal lines in Fig. 5. On the basis of these speculations, it is supposed that the light exposure of the annealed film alters the state from X to Y as x1 + i2 + z1 + y t . However, a reverse process y 1 + i4 + zi + x 1 is also possible in this case. The two processes might occur depending on the photon energy of the illumination. It is plausible that the latter process becomes relatively dominant as the photon energy decreases, since EYZ < E,, . Thus, an effect of the lessbandgap illumination after the bandgap exposure is, in consequence, to transfer the states from Y to X. This consideration explains the thermally-assisted optical bleaching in Fig. 3 and the corresponding refractiveindex quenching in Fig. 1. This model makes clear the relationship between the dynamical and reversible changes. In the reversible process, the states are transferred from Y to X surmounting the barrier EB by the annealing. In the dynamical, the states are transferred to X via the well Z by the less-bandgap illumination with some thermal energy. It is noted here that, since the excited carriers might diffuse a fairly long distance [8], they do not always relax to their original wells. The band-tail states proposed in [4] can be interpreted as the states in the well Y. 4. SUMMARY Role of the less-bandgap illumination in amorphous As2Sa films has been studied. It has been shown that the photo-induced phenomena can be understood in terms of a certain configurational diagram. This diagram explains also the characteristics of the reversible changes.
204
DYNAMICAL CHANGES IN AMORPHOUS As&
It is noted here that the dynamical change is observed also in amorphous GeSa and that this effect can be applied to novel optical devices. Acknowledgements - The author thanks Professors A.
Odajima, T. Sakuma and Y. Ohtsuka for the fruitful discussion. REFERENCES 1.
2.
J.P. DeNeufville, Photostmctural transformations in amophous solids, in B.O. Seraphin (ed.), Optical Properties of Solids -New Developments, North-Holland, Amsterdam (1975). U. Strom & T.P. Martin, Solid State Commun. 29, 527 (1979).
FILMS
Vol. 34, No. 3
3.
M.L. Slade & R. Zallen, Solid State Commun. 30,
4. 5.
K. Tanaka, Solid State Commun. 28,541 (1978). K. Tanaka & Y. Ohtsuka. Thin Solid Films 57. (1979). K. Tanaka, Appl. Phys. Lett. 34,672 (1979). F. Kosek & J. Taut, Czech. J. Phys. B20,94 (1970). N.F. Mott, Phil. Mag. 36,979 (1977). S.J. Hudgens & M. Kastner, Proc. 7th Int. C0n.f Amorph. Liq. Semicond., p. 622 (1978).
6. 7.
;:
357 (1979).
Optical properties of amorphous AsaSs films, which have been illuminated well by bandgap light in advance, can be changed dynamically by exposing to less-bandgap light. This dynamical change has been studied in connection with its plausible relation to the reversible photo-induced change. It has been found that these changes have intimate connections with each other, and can be explained by a certain configurational dia_ gram in a coherent fashion.
2. EXPERIMENTS
1. INTRODUCTION IN AMORPHOUS CHALCOGENIDE FILMS, photostructural transformations [l] have been extensively studied so far. It is well known that the transformations are induced by bandgap illumination, and are associated with certain changes in optical properties, i.e. so-called photodarkening and related refractive-index changes. DeNeufville [I] has classified the phenomena into reversible and irreversible processes, according to whether a heat treatment can or cannot restore the initial structural states. Origin of the irreversible change has been supposed to be a kind of photo-polymerization process [l-3] , whereas the details of the reversible are still not understood. In a previous paper [4], the present author has reported that refractive index of an amorphous Ass& film, which has been illuminated sufficiently by bandgap light beforehand, is dynamically quenched by exposing to radiation with energy below the bandgap. The refractive index of the specimen quenched by the less-bandgap illumination can be enhanced by the bandgap illumination, and the quenching and enhancement can be repeated many times. On the basis of these observations, it has been supposed that the features of the dynamical refractive-index change can be accounted for essentially with a kinetics of some band-tail states. Characteristics of the reversible and dynamical changes are qualitatively similar, but for the fact that in the case of the former the change is recovered by a thermal annealing, whereas in the case of the latter by an optical exposure. Therefore, it is important to clarify the relationship between the reversible and dynamical photo-induced optical changes. This is a purpose of the present paper.
Amorphous AS& fdms were prepared by vacuumevaporation technique under conditions described previously [4]. The samples were, then, annealed at the reversible temperature [l] which is slightly below the glass transition temperature for the case of A& [5] . Prism-coupling technique was utilized for the measurements of refractive index at a photon energy of 1.96 eV and its dispersion [6]. Optical transmittance was investigated also in this study. 3. RESULTS AND DISCUSSION Figure 1 shows a typical result of refractive-index changes in an As& film. For exciting the sample, two light beams of photon energy of 1.96 and 2.8 1 eV having intensities of lo3 and 10’ W cmq2, respectively, were employed. The photon energies of the beams are lower or higher than optical bandgap energy Ego of about 2.4 eV [7]. In addition, it is noted that photon densities absorbed by the specimen are 102’ - 10z2 cmw3see-’ for both the beams. When the specimen is exposed to the 2.81 eVlight, the refractive index increases monotonically from no to a saturation value n1 as a result of the reversible photostructural change [ 1,4] . If the bandgap radiation is ceased, then a spontaneous relaxation to ns results [4] . The magnitude n2 is stable at least one month if the sample is stored in the dark. However, illumination of the less-bandgap light quenches the magnitude n2 to a steady state value n3. It should be noted here that the magnitude n3 is attained also with illumination of the 1.96 eV-light to the annealed specimen as shown by a dashed line in Fig. 1. Note that the magnitudes n2 and n3 are independent of the intensities of the light. As is shown in Fig. 2, the magnitudes of the dynamical refractive-index change n2 -n3 increase with
* This work was partially supported by the Ministry of Education Grant-in-Aid for Scientific Research. 201
202
DYNAMICAL CHANGES IN AMORPHOUS As& FILMS
Vol. 34, No. 3
//---/ “I.96 ..’
1.96eV
eV- light
2.5 I min
photon energy I
time
Fig. 1. Time dependence of photo-induced changes in refractive index by switching on and off the irradiations of 1.96 and 2.81 eV in an annealed As& film of 0.3 pm thick. A dashed line shows a change induced by 1.96 eV-light.
of probing
light
(eV)
Fig. 2. Dependences of n2 -n3 on the photon energy of probing light. Optical bandgap energy is indicated by -5~ -
IO2
2.81 or 1.96 eV A at 77°K
photon energy of excitotionleV)
8 photon
energy
(eV)
Fig. 3. Optical transmittances of an Ass& film after various treatments; annealing and exposure to 1.96 and 2.81 eV-light at room temperature and 77 K. The measurements are carried out at room temperature. The related refractive indices defined in Fig. 1 are written in. probing photon energy in a range of 1.5 - 2.2 eV. Therefore, it is expected [4] with the aid of Kramers-Kronig relations that related absorption charges exist mainly in the photon-energy region higher than 2.2 eV. Figure 3 shows transmission spectra of the specimen after various treatments. The effects induced by the illumination of 2.81 and 1.96 eV-light at room temperature are indicated by solid and dotted arrows, respectively. It is clear that the dynamical refractiveindex change from n2 to n3 accompanies a shift of the optical absorption edge to higher photon energy. Furthermore, the optical absorption edge shifts to lower photon energy in accord with the refractive-index change from no to n3. It should be mentioned that the modifications in the transmission spectra are parallel shifts of the absorption edge. These characteristics are qualitatively similar to those observed in the reversible
Fig. 4. Refractive-index changes at 1.96 eV and absorption-edge shifts with light exposure of various photon energies. The physical quantities on the vertical axes are plotted as the differences from those of the annealed specimen. photodarkening, which is the change induced by the bandgap illumination of the annealed film. As is shown in Figs. 1 and 3, the photon energy of the last exposure determines the film character completely, irrespective of the previous illumination history. That is, the optical properties of the film are determined uniquely by the photon energy of the last exposure. Figure 4 shows the dependence of the optical properties on the photon energy of the excitation. It is apparent that the refractive-index change and the absorption-edge shift are correlated well with each other. When the photon energy of the excitation is greater than the optical bandgap energy Ego, these are nearly independent of the photon energy of the excitation. Otherwise, the change and the shift decrease with the photon energy. It is noted that an annealing treatment at the reversible temperature for all the illuminated films produces a unique physical state, irrespective of the previous illumination history. It is also noted that, since absorption coefficients [7] become smaller with
Vol. 34, No. 3
DYNAMICAL CHANGES IN AMORPHOUS As& FILMS
configuration
Fig. 5. Proposed configurational diagram. photon energies, it seems practically impossible to obtain the data below 1.96 eV. It is known [ 1 ] that illumination of the bandgap light at 77 K reveals larger absorption-edge shift. It is interesting that the similar change is observed with the illumination of the 1.96 eV-light at 77 K as shown by a dashed line in Fig. 3. (Note that the measurements are performed at room temperature.) Therefore, it is supposed that the optical bleaching at room temperature with the irradiation of the less-bandgap light of the film, which was illuminated beforehand by the bandgap light, is assisted essentially with thermal energy of 20 w 30 meV. The effects induced by bandgap and less-bandgap illumination can be understood in terms of a configurational energy diagram illustrated in Fig. 5. The ground level consists of a double well X and Y, and the excited level has a single well 2. In the present speculation, it is essential that the minimum of the well 2 is located above the well Y. The wells X and Y are separated by a potential barrier of Es. Further, there is an energy difference ExY between minima of X and Y. These magnitudes are estimated as EB = 0.5 - 1 eV and ExY = 0.01 - 0.1 eV with results of annealing kinetics. It is supposed that E,, and EYZ N Ego as well as Exz E yz ~0.1 eV. In amorphous specimens, it is very natural to imagine that potential fluctuates locally, thus the precise shape and magnitude of the wells might vary from site to site. In this model, it is not always necessary that transition from Z to X or Y is radiative. For a specimen annealed at the reversible temperature, it is reasonable to assume that all the states are in the well X. According to the Frank-Condon principle, it is expected that the bandgap exposure of the
203
annealed film at low temperatures produces a transition x,, + jr -+ ze + y,, . Reexcitation of the states in the well Y is possible, however this proceeds asyo + i3 + z. +yo. Therefore, almost all the states are transferred from X to Y by the bandgap illumination at low temperatures. Considering the spread of wavefunctions at the well X and the fluctuation in the gap energy Exz, we suppose that illumination of the less-bandgap light at low temperatures produces similar processes with lower efficiencies. Thus, by the optical exposure at low temperatures, the gap decreases from Exz to EyZ. The ensemble average of the difference Exz - EYZ corresponds to the edge shift at 77 K in Fig. 3. On the contrary, the light exposure at room temperature has a distinct effect. With the aid of the thermal energy, equilibrium states in the well X and Y are not confined to the bottoms. In addition, since the relaxation time in the well Z is lo-l2 set and transition probability from Z to X or Y is lOa set-’ [8] , it is reasonable to assume a thermal equilibrium level for the well Z. These hypothetical levels are indicated tentatively by horizontal lines in Fig. 5. On the basis of these speculations, it is supposed that the light exposure of the annealed film alters the state from X to Y as x1 + i2 + z1 + y t . However, a reverse process y 1 + i4 + zi + x 1 is also possible in this case. The two processes might occur depending on the photon energy of the illumination. It is plausible that the latter process becomes relatively dominant as the photon energy decreases, since EYZ < E,, . Thus, an effect of the lessbandgap illumination after the bandgap exposure is, in consequence, to transfer the states from Y to X. This consideration explains the thermally-assisted optical bleaching in Fig. 3 and the corresponding refractiveindex quenching in Fig. 1. This model makes clear the relationship between the dynamical and reversible changes. In the reversible process, the states are transferred from Y to X surmounting the barrier EB by the annealing. In the dynamical, the states are transferred to X via the well Z by the less-bandgap illumination with some thermal energy. It is noted here that, since the excited carriers might diffuse a fairly long distance [8], they do not always relax to their original wells. The band-tail states proposed in [4] can be interpreted as the states in the well Y. 4. SUMMARY Role of the less-bandgap illumination in amorphous As2Sa films has been studied. It has been shown that the photo-induced phenomena can be understood in terms of a certain configurational diagram. This diagram explains also the characteristics of the reversible changes.
204
DYNAMICAL CHANGES IN AMORPHOUS As&
It is noted here that the dynamical change is observed also in amorphous GeSa and that this effect can be applied to novel optical devices. Acknowledgements - The author thanks Professors A.
Odajima, T. Sakuma and Y. Ohtsuka for the fruitful discussion. REFERENCES 1.
2.
J.P. DeNeufville, Photostmctural transformations in amophous solids, in B.O. Seraphin (ed.), Optical Properties of Solids -New Developments, North-Holland, Amsterdam (1975). U. Strom & T.P. Martin, Solid State Commun. 29, 527 (1979).
FILMS
Vol. 34, No. 3
3.
M.L. Slade & R. Zallen, Solid State Commun. 30,
4. 5.
K. Tanaka, Solid State Commun. 28,541 (1978). K. Tanaka & Y. Ohtsuka. Thin Solid Films 57. (1979). K. Tanaka, Appl. Phys. Lett. 34,672 (1979). F. Kosek & J. Taut, Czech. J. Phys. B20,94 (1970). N.F. Mott, Phil. Mag. 36,979 (1977). S.J. Hudgens & M. Kastner, Proc. 7th Int. C0n.f Amorph. Liq. Semicond., p. 622 (1978).
6. 7.
;:
357 (1979).